Flutter 根据从第一个 DropdownButtonForm 中 Select 的内容在第二个 DropdownButton 上显示选项

我正在开发一个带有Ffltter的移动应用程序.我正在为申请准备一个注册页面.在注册页面上，我想要获得用户居住的城市和县.我放了两个DropdownButtonForm的这个.

在用户 Select 第一个DropdownButtonForm中的城市后，第二个DropdownButtonForm将具有县选项.我可以用下面的代码做到这一点:

   List<DropdownMenuItem<String>> kahramanmarasDistricts = [
      const DropdownMenuItem(
        value: "Onikişubat",
        child: Text("Onikişubat"),
      ),
      const DropdownMenuItem(
        value: "Dulkadiroğlu",
        child: Text("Dulkadiroğlu"),
      ),
      const DropdownMenuItem(
        value: "Elbistan",
        child: Text("Elbistan"),
      ),
      const DropdownMenuItem(
        value: "Afşin",
        child: Text("Afşin"),
      ),
      const DropdownMenuItem(
        value: "Türkoğlu",
        child: Text("Türkoğlu"),
      ),
      const DropdownMenuItem(
        value: "Pazarcık",
        child: Text("Pazarcık"),
      ),
      const DropdownMenuItem(
        value: "Göksun",
        child: Text("Göksun"),
      ),
      const DropdownMenuItem(
        value: "Andırın",
        child: Text("Andırın"),
      ),
      const DropdownMenuItem(
        value: "Çağlayancerit",
        child: Text("Çağlayancerit"),
      ),
      const DropdownMenuItem(
        value: "Nurhak",
        child: Text("Nurhak"),
      ),
      const DropdownMenuItem(
        value: "Ekinözü",
        child: Text("Ekinözü"),
      ),
    ];
    
    List<DropdownMenuItem<String>> diyarbakirDistricts = [
      const DropdownMenuItem(
        value: "Bağlar",
        child: Text("Bağlar"),
      ),
      const DropdownMenuItem(
        value: "Bismil",
        child: Text("Bismil"),
      ),
      const DropdownMenuItem(
        value: "Çermik",
        child: Text("Çermik"),
      ),
      const DropdownMenuItem(
        value: "Çınar",
        child: Text("Çınar"),
      ),
      const DropdownMenuItem(
        value: "Çüngüş",
        child: Text("Çüngüş"),
      ),
      const DropdownMenuItem(
        value: "Dicle",
        child: Text("Dicle"),
      ),
      const DropdownMenuItem(
        value: "Egir",
        child: Text("Eğir"),
      ),
      const DropdownMenuItem(
        value: "Ergani",
        child: Text("Ergani"),
      ),
      const DropdownMenuItem(
        value: "Hani",
        child: Text("Hani"),
      ),
      const DropdownMenuItem(
        value: "Hazro",
        child: Text("Hazro"),
      ),
      const DropdownMenuItem(
        value: "Kayapınar",
        child: Text("Kayapınar"),
      ),
      const DropdownMenuItem(
        value: "Hocaköy",
        child: Text("Hocaköy"),
      ),
      const DropdownMenuItem(
        value: "Kulp",
        child: Text("Kulp"),
      ),
      const DropdownMenuItem(
        value: "Lice",
        child: Text("Lice"),
      ),
      const DropdownMenuItem(
        value: "Silvan",
        child: Text("Silvan"),
      ),
      const DropdownMenuItem(
        value: "Sur",
        child: Text("Sur"),
      ),
      const DropdownMenuItem(
        value: "Yenişehir",
        child: Text("Yenişehir"),
      ),
    ];
List<DropdownMenuItem<String>> citys = [
  const DropdownMenuItem(
    value: "Kahramanmaraş",
    child: Text("Kahramanmaraş"),
  ),
  const DropdownMenuItem(
    value: "Diyarbakır",
    child: Text("Diyarbakır"),
  ),
];
// ...
SelectCityDropDownField(),
SelectDistrictDropDownField(),

  Widget SelectCityDropDownField() {
    return Theme(
      data: Theme.of(context).copyWith(
        colorScheme: ThemeData().colorScheme.copyWith(
              primary: Colors.red,
            ),
      ),
      child: DropdownButtonFormField(
        decoration: InputDecoration(
          prefixIcon: const Icon(
            Icons.location_city,
            color: Color(0xFFCB3126),
          ),
          border: OutlineInputBorder(
            borderRadius: BorderRadius.circular(50),
          ),
          enabledBorder: OutlineInputBorder(
            borderRadius: BorderRadius.circular(50),
            borderSide: const BorderSide(
              color: Color(0xFFCB3126),
            ),
          ),
        ),
        items: citys
            .map(
              (e) => DropdownMenuItem<String>(
                value: e.value,
                child: e.child,
              ),
            )
            .toList(),
        onChanged: (value) {
          setState(() {
            selectedCity = value.toString();
          });
        },
      ),
    );
  }

  Widget SelectDistrictDropDownField() {
    return Theme(
      data: Theme.of(context).copyWith(
        colorScheme: ThemeData().colorScheme.copyWith(
              primary: Colors.red,
            ),
      ),
      child: DropdownButtonFormField(
        decoration: InputDecoration(
          prefixIcon: const Icon(
            Icons.location_city,
            color: Color(0xFFCB3126),
          ),
          border: OutlineInputBorder(
            borderRadius: BorderRadius.circular(50),
          ),
          enabledBorder: OutlineInputBorder(
            borderRadius: BorderRadius.circular(50),
            borderSide: const BorderSide(
              color: Color(0xFFCB3126),
            ),
          ),
        ),
        items: districtsItem(),
        onChanged: (value) {
          setState(() {
            selectedDistrict = value.toString();
          });
        },
      ),
    );
  }

  districtsItem() {
    switch (selectedCity) {
      case "Kahramanmaraş":
        return kahramanmarasDistricts;

      case "Diyarbakır":
        return diyarbakirDistricts;
    }
  }

我对一些事情有意见.问题是这样的:例如，让我们 Select Kahramanmaraş人的省份，并 Select Elbistan作为区.然后，当我将城市更改为Diyarbakır时，它会给出一个错误.换句话说，当我在 Select 了地区之后 Select 了一个不同的省份时，我会得到一个错误.这是错误的；

The following assertion was thrown building Builder(dirty, dependencies: [_FocusMarker]):
There should be exactly one item with [DropdownButtonFormField]'s value: Çermik. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 939 pos 15: 'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1'

我怎么才能解决这个问题呢？首先要感谢你的帮助.